cmd/go/internal/modget: consolidate Load entrypoints
    
    This change replaces ImportPaths, ImportPathsQuiet, LoadALL, and
    LoadVendor with a single LoadPackages function, with a LoadOpts struct
    that more clearly documents the variations in behavior.
    
    It also eliminates the cmd/go/internal/load.ImportPaths function,
    which was undocumented and had only one call site (within its own
    package).
    
    The modload.LoadTests global variable is subsumed by a field in the
    new LoadOpts struct, and is no longer needed for callers that invoke
    LoadPackages directly. It has been (temporarily) replaced with a
    similar global variable, load.ModResolveTests, which can itself be
    converted to an explicit, local argument.
    
    For #37438
    For #36460
    Updates #40775
    Fixes #26977
    
    Change-Id: I4fb6086c01b04de829d98875db19cf0118d40f8c
    Reviewed-on: https://go-review.googlesource.com/c/go/+/255938
    Trust: Bryan C. Mills <bcmills@google.com>
    Trust: Jay Conrod <jayconrod@google.com>
    Run-TryBot: Bryan C. Mills <bcmills@google.com>
    TryBot-Result: Go Bot <gobot@golang.org>
    Reviewed-by: Michael Matloob <matloob@golang.org>
    Reviewed-by: Jay Conrod <jayconrod@google.com>